It is a constant endeavor to find ways of increasing the pinout density of integrated circuits (ICs). This is particularly important in ICs with high pin counts and relatively small packages. Solder ball connection (SBC) technology was developed to satisfy this growing need. In general the number of electronic circuits that can be manufactured per unit area of silicon or board space has increased dramatically in recent years. This increase in circuit density has produced a corresponding increase in the number of connections required between the various electronic circuits, integrated circuit modules, and boards to facilitate the manufacture of more complex products. High density integrated circuit modules such as microprocessors are typically housed in a protective package such as a pin grid array (PGA) module, that provides a means of connecting to other electronic circuits on a printed circuit board. PGA modules have an array of metal pins on the bottom side of the package. The
PGA module is often connected and disconnected through these pins to a test card to evaluate the individual electronic circuit components before they are assembled into a final product. Connector sockets are available for PGA modules to facilitate the connection and disconnection of the PGA module for service, upgrade, or testing requirements.
Solder ball connection (SBC) modules have recently been developed to address the need for more signal and ground connections, higher density packaging, and compatibility with surface mount technology processes. SBC modules are different than PGA modules in that the SBC module uses solder balls instead of metal pins to provide for interconnections between the module and the printed circuit board. Whereas PGA pins are spaced 0.10 inches apart, the solder balls are typically arranged in an area array pattern with 0.05 inch spacing. This close spacing makes it difficult to use previous interconnector technologies for SBC sockets. Thus, although SBC modules have service, upgrade, burn-in and testing requirements similar to PGA modules, SBC interconnecting sockets are not commonly available.
Because of the SBC pin densities, a specialized technology such as that generally used in elastomeric connectors is one of the various interconnection technologies that can be used for an SBC socket. The elastomeric connector provides a conductive path through an elastomeric material to connect the solder balls on the SBC module to the contact pads on the printed circuit board. A wide variety of elastomeric connectors are available that use conductive particles or conductive wires embedded in the elastomer material. The electrical, mechanical, and thermal performance of the elastomeric connector can vary significantly depending on the size, shape, orientation, and density of the conductive particles as well as the properties of the elastomer material . An SBC socket has unique requirement which need to be satisfied if the socket is to be useful, reliable and durable. Elastomeric connectors that are suitable for area array interconnections have previously been described.
U.S. Pat. No. 4,998,885, issued Mar. 12, 1991 to Beaman et al., is directed to an elastometer area array interposer. It provides for an elastomeric interposer surrounding fine metal wires which extend through the elastomeric materials permanently bonded to a rigid wiring substrate. It is useful for electrically connecting two substrates having high density interconnections. The structure described is comprised of conductive wires embedded in an elastomer material that are permanently bonded to the rigid wiring substrate. The contact interface is comprised of ball shaped gold wire conductors surrounded by an elastomer material. It is specifically adapted to interconnecting with flat gold plated pads on the surface of the mating substrate. The ball shaped contact surface is not large enough for use with an SBC type contact. SBC interconnection with this type of a ball shaped contact surface would cause excessive degradation of the connecting wires and ball shaped contact and limit the durability and reliability of the interface for socket applications.
U.S. Pat. No. 5,371,654, issued Dec. 6, 1994 to Beaman et al., is directed to a structure for packaging electronic devices, such as semiconductor chips, in a three dimensional structure. The structure includes a multilayer wiring substrate for X-Y connections along with an elastomeric connector for Z axis connections. The elastomeric connector described is specifically adapted to a three dimensional packaging approach and does not require a contact interface with high durability. Also, the contact interface in this patent is comprised of an elastomeric connector having gold wire conductors mated to flat gold plated pads on the surface of the multilayer wiring substrate. The contact interface on the elastomeric connector would not be compatible for connection to the solder ball on an SBC module.
The present invention provides solutions to the unique SBC module interconnecting problems. The following interconnector requirements should be considered in order to satisfactorily connect to a module whose pinouts are solder ball connections.
Firstly, the solder ball connections have irregular ball sizes. Additionally, the printed circuit board to which the module is connected has planarity variations of its contact pads. Thus, the interconnector contact interface should have high compliance to enable it to yield elastically when contact forces are applied.
Secondly, the SBC interconnector should have a contact metallurgy such that it does not form a high resistance intermetallic layer with the solder balls even for high temperature applications.
Thirdly, the interconnector should be such as to minimize deformation of the solder ball while the SBC module is connected to the interconnecting structure.
Fourthly, the SBC interconnector contact interface should be of high durability to allow multiple repeated connections and disconnections of the SBC module without degradation.
In addition, the SBC interconnector should act as a clamping fixture so as to provide consistent pressure between the SBC module and the interconnector contacts. Preferably, the fixture should include a means of aligning the corresponding contacts on the interconnector with the solder balls on the module and also with the connecting article.